The present invention relates to structural adhesive compositions. More particularly, the invention relates to improving environmental resistance and low temperature properties of acrylic structural adhesive compositions.
Acrylic structural adhesive compositions are well-known articles of commerce which are extensively used commercially for bonding metal and plastic materials. The load-bearing and stress-relieving properties of structural adhesives, as well as their bond strength, which can exceed the strength of the materials which are being bonded, make these adhesives attrative alternatives to or replacements for mechanical methods, such as riveting or spot welding, of joining materials, especially where it is preferable to distribute load stresses over larger areas rather than to concentrate such stresses at a few points. Their use can reduce or eliminate costly finishing operations necessitated by mechanical joining methods, present a more pleasing exterior and at least reduce the possiblity of corrosion of assemblies containing one or more metal components. Additionally, they can be used to bond a diversity of metals without extensive surface preparation. For example, Zalucha et al U.S. Pat. No. 4,223,115 and Briggs et al U.S. Pat. No. 3,890,407 disclose acrylic structural adhesive compositions which are effective bonding materials for oily metal surfaces.
Despite the attractiveness of acrylic structural adhesives, they are not without deficiency. For example, an application area for such adhesives is in the bonding of lightweight metal and plastic materials in the fabrication of vehicle bodies and component parts. A requirement in many applications is for satisfactory adhesion at temperatures below -25.degree. C. While acrylic structural adhesives provide excellent bond characteristics at ambient temperatures, they, as with almost all adhesives, become embrittled and suffer significant loss of adhesion and impact resistance at these low temperatures. There is a demonstrated need for acrylic structural adhesives with good low temperaure properties.
The use of elastomers in acrylic structural adhesives to improve low temperature properties, such as impact resistance, is well known. While both solid gum and liquid elastomers can be utilized for this purpose, the limited solubility and greater incompatibility of the solid elastomers can restrict their use, especially at higher levels of elastomer concentration. Liquid elastomers, especially olefinic-terminated liquid elastomers are attractive candidates for use with free radical-cured acrylic structural adhesives. Such elastomers have been commercially available for a number of years and have been widely used as toughening agents for polyester molding compounds.
Representative of the olefinic-terminated liquid elastomers are the methacrylate-terminated polybutadiene homopolymers and copolymers which can be prepared by esterification reaction of glycidyl methacrylate with carboxyl-terminated polybutadiene, poly(butadiene-(meth)acrylonitrile) or poly(butadiene-(meth)acrylonitrile-styrene). Investigations into the use of such methacrylate-terminated liquid elastomers for improving low temperature properties of free radical-cured acrylic structural adhesives demonstrated that these compounds do indeed improve low temperature shear, impact and peel strengths with only slight sacrifice in room temperature shear strength. Unfortunately, the acrylic structural adhesives containing such liquid elastomers exhibit a severe sensitivity to moisture as evidenced by losses of up to 50 percent of original bond strength after as little as two weeks exposure to aggresive environments such as boiling water and salt spray.
Continued investigations into the use of olefinic-terminated liquid elastomers resulted in the discovery that environmental resistance of free radical-cured acrylic structural adhesives containing olefinic-terminated liquid elastomers is unexpectedly and dramatically improved with no significant loss in low temperature and ambient temperature adhesion properties if the secondary hydroxyl group(s) which such liquid elastomers contain are reacted with monoisocyanate compounds. It has been found that the retention of initial bond strength after two weeks of environmental cycling increased from 58 percent in the case of an unmodified liquid elastomer to 97 percent when isocyanate capping was carried out at 95 percent of theory. In addition, adhesion to galvanized steel is unexpectedly enhanced when the esterification of the carboxyl-functional liquid elastomer is foreced to completion prior to isocyanate capping. For example, when the liquid elastomer has a residual acid value of 3-5 prior to capping, torsional impact strengths in the range of 30 in. lbs. are observed; whereas if the esterification is carried out to near completion (acid no. 1.0), torsional impact strengths of greater than 50 in. lb. are observed.